This invention relates to superconducting windings in general and more particularly to an evacuating device for generating an insulating vacuum around the superconducting winding of the rotor of an electric machine with an interior which is surrounded by the winding and is permeated by the magnetic field of the winding.
The superconducting field winding of the rotor of an electric machine, for instance, of a turbo-generator, is generally cooled with liquid helium. In order to limit the heat transfer from the outside to this deeply cooled winding, it is advantageous to surround the winding with an insulating vacuum which should in general be better than 10.sup.-6 mbar. While all gas components with the exception of the helium are bound to the surface portions of the rotor at the low helium temperature, due to a cyro pump effect because of the cooling with liquid helium, the insulating vacuum is worsened by helium leaks of the helium carrying rotor parts, especially at soldered or welded joints. For the helium carrying parts of the machine, the leakage rate would have to be less than 10.sup.-10 mbar. l/sec with an operating time of the machine of one year, in order for it to be unnecessary to pump helium out of the insulating vacuum. Such low leakage rates can be achieved, however, only with very large expenditure with respect to design costs in the choice of materials and in fabrication costs. In general, the leakage rates for the helium carrying parts are around 10.sup.-8 mbar. l/sec. With an extended operating time of, say, one year, an excessively large pressure increase of the insulating vacuum is then obtained. With this leakage rate, the pressure in a vacuum volume of 1000 l rises, for instance, to 10.sup.-4 mbar in less than a year. It is therefore necessary to pump the helium gas out of the vacuum space during the rotating operation. For this purpose, for instance, a centrally arranged evacuating tube, via which the vacuum space can be evacuated by means of a pump arranged outside the rotor, is generally provided in the terminal head of the rotor of the electric machine. To connect the rotating evacuating tube to the stationary pump line of the pump, a rotating sealing element must then also be provided at the terminal head. Such sealing elements, however, can be made for the required high insulating vacuum only with a relatively large expenditure, and require maintenance.